(1) Field of the Invention
The present invention relates to cooling rolls for producing rapidly solidified metal strip sheets. More specifically, the invention is aimed at advantageously producing sound strip sheet products by reducing to the utmost a heat crown inevitably occurring at the outer peripheral surface of the cooling roll during cooling-solidification step of a molten metal.
(2) Related Art Statement
A technique for continuously obtaining rapidly solidified metal strip sheets by directly feeding a molten metal to a surface of a cooling roll and rapidly cooling and solidifying it has widely been used as a method for producing amorphous alloys by means of a single roll or a method of rapidly solidifying a liquid by using double rolls.
However, since the molten metal is cooled to not more than its solidification point or not more than its crystallization temperature by rapidly extracting heat from the molten metal, the temperature of the outer peripheral surface of the roll with which the molten steel is brought into contact increases, and the cooling roll consequently thermally expands. At that time, a temperature gradient is developed in an axial direction of the roll between a contacting portion and a non-contacting portion with the molten metal, so that the roll surface is deformed in a barrel-like shape having a larger curvature to form a so-called heat crown.
In the rapidly liquid-solidifying method using a single roll, a nozzle having a narrow slit-like shape is generally used, and its tip is approached to the surface of the roll at a narrow spatial distance range of about 0.1 to 0.5 mm. Thus, when the dimension of the nozzle slit, the peripheral speed of the roll, and a pressure for injecting the molten metal are set constant, the thickness of the strip sheet is largely influenced by the gap between the nozzle and the roll. Therefore, if a heat crown is formed at the outer peripheral surface of the roll, the gap between the nozzle and the roll becomes narrower at the widthwise central portion of the strip sheet. Accordingly, there occurs an inconvenience that the thickness of the strip sheet is smaller at its central portion and larger at the end portions.
In order to solve thickness variations in strip sheets due to the above heat crown, Japanese Patent Application Laid-open Nos. 56-68,559, 59-54,445, 57-112,954 and 58-135,751 proposed techniques by which a temperature distribution is uniformized by varying cooling power between the central portion and the end portions of the roll with due consideration of number, dimension and shape of cooling channels to enhance the cooling power at the widthwise central portion of the sleeve as compared with that at the end portions thereof, thereby preventing occurrence of the heat crown. Each of these techniques may be called a method of increasing an amount of heat to be extracted from the widthwise central portion of the roll by relatively increasing an amount of cooling water or a cooling area at the widthwise central portion of the sleeve as compared with the end portions thereof.
However, since the above method is obliged to exchange the cooling roll when the width of strip sheets to be produced varies, and as mentioned later, even if the temperature distribution is made uniform in the roll axial direction, this does not mean that thermal expansion is uniformized and the crown heat is diminished.
Japanese Patent Application Laid-open No. 59-229,263 proposed a technique of mechanically grinding off thickness difference, due to the thermal expansion, between the widthwise central portion and end portions of the roll. However, although such a technique is not impossible as an idea basis, a large size equipment provided with a precision machine is not only necessary, but also this technique is an impractical method necessitating a precision polishing of the rolled surface during pouring the molten metal. Thus, it is actually inapplicable.
Japanese Patent Publication No. 60-51,933, now Japanese Pat. No. 1,327,971 (U.S. patent application No. 115,517, filed on Jan. 25, 1980, now U.S. Pat. No. 4,307,771) proposed a technique in which cooling channels are formed inside a metal sleeve in parallel with a roll axial direction to make the thermal expansion in the roll radial direction constant and to lessen the heat crown. In this technique, it is necessary to provide a plurality of the cooling water channels in parallel with the roll axial direction and spaced at an interval in a circumferential direction, and a cooling water stay portion on a water feed side and a cooling water stay portion on a water discharge side in axial ends of a wheel. Therefore, a fixing mechanism naturally becomes necessary at the wheel central portion.
However, this technique places its emphasis upon a radial heat expansion of the wheel and an accompanying radial thermal stress only, but it utterly fails to consider importance of the thermal expansion in the roll axial direction which the present invention makes much of. Furthermore, the fixing mechanism at the wheel central portion becomes complicated and a high dimensional precision is also required in the fitting portions between the inner surface of the wheel and the shaft end portions. Thus, extremely precision machining becomes necessary. In addition, this technique has a disadvantage that heat expansion is not improved to a satisfactory degree despite of the high machining technique and high cost.
As mentioned above, in the case of the single roll method, the cooling roll is deformed in a barrel-like shape during the casting process, and a gap between the nozzle and the roll becomes narrower at the widthwise central portion of the strip sheet. As a result, the products becomes thinner at the central portion thereof.
Needless to speak of amorphous alloy strip sheets, it is extremely difficult to relatively correct the thickness distribution of the strip sheet in the widthwise direction during a succeeding rolling, etc.
In the above-mentioned Japanese Patent Publication No. 56-68,559 and Japanese Patent Application Laid-open Nos. 59-54,445, 57-112,954 and 58-135,751, control is made such that the temperature distribution in the roll axial direction may be uniformized over the whole width of the strip sheet by appropriately devising the water cooling structure inside the cooling roll. In other words, these techniques are based on the assumption that if the temperature distribution is uniform, the amount of the thermal expansion becomes uniform so that no heat crown occurs.
However, it was confirmed through close examinations of the heat crown-occurring mechanism in experiments and computer simulations that this assumption is extremely insufficient and that heat crown cannot be suppressed to a satisfactorily low degree by uniformly controlling the temperature distribution. That is, it was experimentally and thorough the simulations that when rapidly solidified metal strip sheets were cast by using a cooling roll as shown in FIG. 2 in which heat insulating portions are formed in a roll axial direction by cutting deep grooves in the sleeve apart by 3 mm outside a strip sheet of 100 mm width to make a heat flow flux from the surface of the sleeve flow in the roll radial direction only, the temperature on the surface of the sleeve is highly uniform inside the deep grooves. However, the amount of the thermal expansion and the thickness distribution of the rapidly solidified metal strip sheet produced as measured at the same time were almost the same as in a case using a rapidly cooling roll of an ordinary type in which the sleeve surface temperature becomes higher at the center in the roll axial direction. Thus, extremely insufficient results only could be obtained.
From the above experimental facts, it was concluded that the heat crown problem could not effectively be solved by the prior art techniques having noted the surface temperature of the roll only.